HIV-1 has infected over 57 million and killed over 21 million individuals worldwide since the beginning of the epidemic. More than 95 percent of HIV-1 infected individuals live in developing countries and have no access to antiretroviral treatment. The need for a vaccine that protects against HIV-1 infection or attenuates disease has never been more urgent. Understanding the correlates of protective immunity is a logical first step in vaccine development. In the case of HIV-1 immunity, accumulating data have shown a central role of HIV-specific cellular immune responses in controlling viral replication. The immune activity generated in acute infection in particular is thought to be critical for establishing the ultimate speed of progression to disease. Recent studies in the macaque model suggest that cytotoxic T lymphocyte (CTL) responses directed against the early expressed regulatory protein Tat play a central role in the SIV-specific primary immune response. In humans, much less is known about the role of the cellular immune responses directed against the regulatory protein Tat and Rev and the accessory proteins Vpr, Vpu and Vif, as the analysis of HIV-1-specific immune responses has been dominated by studies of virus-specific immune responses directed against structural HW-1 proteins, using HIV-1 laboratory strain or consensus sequences. The regulatory and accessory HIV- 1 proteins may however represent important targets for cellular immune responses, due to their very early expression during the viral life cycle (Rev and Tat) and their important function during viral replication The proposed project therefore focuses on the comprehensive analysis of HIV-1-specific CTL responses directed against the autologous virus in order to determine the contribution of responses directed against these HIV-1 proteins to the total virus-specific CTL response and to assess their influence on viral sequence variation, using two unique cohorts of HIV-1 infected persons: (1) individuals with acute HIV-1 infection and (2) individuals who control plasma viremia in the absence of antiretroviral therapy. The following specific aims should be addressed: (1) Comprehensive characterization of CTL responses directed against the regulatory and accessory HIV-1 proteins using autologous virus sequences; (2) Longitudinal assessment of immune selection pressure mediated through CTL responses directed against accessory, regulatory and structural HIV-1 proteins.